(CNN) -- Most Americans blame emissions from cars and industrial plants as the primary cause of global warming and believe the United States should reduce levels even if other countries don't, a survey shows.

Fifty-six percent of poll respondents said the phenomenon of global warming has been proven, and can be largely blamed on human endeavors, such as power plants and factories, according to the CNN/Opinion Research Corp. poll.

In comparison, 21 percent of those surveyed claimed global warming problems are caused either by natural changes or are unproven.

Sixty-six percent of Americans believe the United States should do what it can to reduce global warming, even if other nations ignore it. This compares with 52 percent of respondents who believed that way in 2001.